Steam generators, especially steam generators of pressurized-water nuclear reactors, comprise tubes bent in the form of a U and having two branches crimped at their ends in a tube plate. The tubes are arranged according to adjacent plane layers. The curved parts of the tubes of the bundle which are juxtaposed in its upper part have radii of curvature different from one another and are placed mutually adjacent to form a structure of substantially hemispherical form, called a bun.
While the steam generator is in operation, pressurized water at high temperatures circulates in the tubes of the bundle and feed water is brought in contact with the outer surface of these tubes, along which it travels in the vertical direction, at the same time heating up and then evaporating, in order to re-emerge in the form of steam in the upper part of the generator.
The circulation of the fluid in contact with the tubes can give rise to vibrations which are liable to cause damage to these tubes if they are not held effectively.
The straight part of the tubes is engaged in spacers arranged at uniform distances from one another over the height of the bundles. These straight parts are therefore held effectively by rigid elements. The curved parts of the tubes of the bundle forming the bun must likewise be held, and this purpose is generally served by the use of spacer bars which are each interposed between two layers of adjacent tubes of the bundle and which are arranged in substantially radial directions of the bun. These spacer bars are usually connected two by two in an articulated manner at one of their ends located towards the inside of the bun and are placed angularly so as to form V-shaped structures.
The outer ends of the anti-vibration bars opposite their articulated ends project relative to the tubes forming the outer layer of the bun and are connected to one another by connection means ensuring retention of the anti-vibration bars.
Various means for connecting the outer ends of the anti-vibration bars, using fastening elements placed on the upper surface of the bun, have been proposed.
It has been proposed, for example, to weld the ends of the anti-vibration bars to curved elements arranged along mid-planes of the bun.
It has also been proposed to fasten the ends of the anti-vibration bars, for example by welding, to substantially parallel hoops resting on the upper surface of the bun. At all events, the fastening of the anti-vibration bars requires complex operations most often employing welds which have to be made in the vicinity of the tubes of the bundle which thus risk suffering some damage. Moreover, the component materials of the anti-vibration bars must be selected to allow their welding and, if appropriate, their treatment after welding, under good metallurgical conditions.
There have also been provided mechanical connection devices making it possible to connect the outer ends of the anti-vibration bars to fastening pieces, such as retaining hoops resting on the outer surface of the bun.
These devices are relatively complex and, for fastening them, require the use of heavy and bulky complementary elements attached to the bun. Furthermore, these devices do not make it possible to ensure exact alignment of the anti-vibration bars and to adjust accurately the play present between two successive anti-vibration bars, between which a layer of tubes is interposed.